Absorption chillers

ABSTRACT

An absorption chiller system is provided. The system includes an evaporator, an absorber, a divider, a recirculating loop, and a primary heater. The evaporator includes a first working fluid inlet and a first working fluid outlet. The absorber includes a second working fluid inlet and a second working fluid outlet. The divider has opposing first and second sides, wherein the first side is in fluid communication with the evaporator and the second side is in fluid communication with the absorber. The recirculating loop connects the first working fluid outlet back to the evaporator, and the primary heater is disposed in thermal communication with the recirculating loop.

BACKGROUND

This invention relates generally to an absorption chiller system. Particularly the invention relates to an absorption chiller system including an external recirculating loop for a working fluid.

Absorption chillers are thermally driven temperature lift devices where heat sources provide the driving thermal energy. Absorption chillers are environmentally friendly and generally use less electrical energy compared to conventional coolers that use electrically driven vapor compressors to provide the primary driving energy. Absorption chillers provide a cooling effect by evaporating a refrigerant in an evaporator. The resultant refrigerant vapor is then combined with an absorbent in an absorber. The absorption chillers may be powered by different means, including natural gas, steam, or waste heat.

Crystallization of the absorbents such as lithium bromide in the absorber is one of the classic technical challenges for commercializing absorption chillers. Membrane-based absorption chillers generally help in reducing the crystallization of the absorbents. Further control of crystallization of absorbents at the absorber, thereby enabling the absorption chillers to operate in a wide temperature range, is an existing need. Embodiments of the present invention relate to the systems and methods that enable further reduction in crystallization of absorbent at the absorber.

BRIEF DESCRIPTION

Briefly, in one embodiment, a system is provided. The system includes an evaporator, an absorber, a divider, a recirculating loop, and a primary heater. The evaporator includes a first working fluid inlet and a first working fluid outlet. The absorber includes a second working fluid inlet and a second working fluid outlet. The divider has opposing first and second sides, wherein the first side is in fluid communication with the evaporator and the second side is in fluid communication with the absorber. The recirculating loop connects the first working fluid outlet back to the evaporator, and the primary heater is disposed in thermal communication with the recirculating loop.

In one embodiment, a system is provided. The system includes an evaporator, an absorber, a divider, a recirculating loop, a primary heater, and a secondary heater. The evaporator includes a first working fluid inlet and a first working fluid outlet. The absorber includes a second working fluid inlet and a second working fluid outlet. The divider can be a hydrophobic porous membrane and has opposing first and second sides, wherein the first side is in fluid communication with the evaporator and the second side is in fluid communication with the absorber. The recirculating loop connects the first working fluid outlet back to the evaporator. The primary heater is disposed in thermal communication with the recirculating loop and the secondary heater is disposed in thermal communication with the evaporator.

In one embodiment, a method is disclosed. The method includes the steps of evaporating at least a part of a first working fluid in an evaporator, passing at least a part of the first working fluid vapor through a divider to an absorber that includes a second working fluid, exiting at least a part of the first working fluid through a first working fluid outlet of the evaporator, heating at least a part of the first working fluid after departing from the first working fluid outlet, and recirculating at least a part of the heated first working fluid back to the evaporator.

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates an absorption chiller system, according to an embodiment of the invention;

FIG. 2 illustrates arrangement of primary heater according to an embodiment of the invention;

FIG. 3 illustrates an absorption chiller system as per an example according to one embodiment of the invention;

FIG. 4 illustrates arrangement of primary heater according to an embodiment of the invention;

FIG. 5 illustrates a Dühring plot as per an example according to one embodiment of the invention; and

FIG. 6 compares the vapor pressures of a conventional system with an absorption chiller, according to one embodiment of the invention.

DETAILED DESCRIPTION

One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

In the following specification and the claims that follow, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

Referring to FIG. 1, a part of an absorption chiller system 10, configured in accordance with an example embodiment, is shown. The system 10 can include an evaporator 12 and an absorber 14. A divider 16 may be disposed between the evaporator 12 and the absorber 14. In one embodiment, the divider 16 may be a membrane. The divider 16 has opposing first 18 and second 20 sides with the evaporator 12 being in fluid communication with the first side 18 and the absorber 14 being in fluid communication with the second side 20. For example, in one embodiment, the evaporator 12 can be coupled to the first side 18 and the absorber 14 can be coupled to the second side 20.

The evaporator 12 may have a first working fluid inlet 22 and a first working fluid outlet 24. The evaporator 12 may be configured to receive a first working fluid (refrigerant) 26 and to produce first working fluid vapor 28. For example, in one embodiment, the first working fluid 26 may be water, and the evaporator 12 may receive liquid water (for example, through first working fluid inlet 22) and may produce water vapor 28. Other candidate first working fluids are discussed below. Liquid first working fluid 26 may circulate through the evaporator 12, and the unevaporated portions are emitted from the evaporator, for example, at the first working fluid outlet 24.

The absorber 14 may have a second working fluid inlet 32 and a second working fluid outlet 34. The absorber 14 may be configured to receive the first working fluid vapor 28 and to combine at least some of that first working fluid vapor 28 with a second working fluid (absorbent) 36. The second working fluid 36 may circulate through the absorber 14, such that the second working fluid is received, for instance, at the second working fluid inlet 32, travels through the absorber, and exits at the second working fluid outlet 34. Given that the second working fluid 36 enters the absorber 14 and then is combined therein with first working fluid vapor 28, the second working fluid 36 entering the absorber has a relatively lesser concentration therein of first working fluid than does the second working fluid 36 inside and exiting the absorber. The second working fluid 36 entering the absorber 14 at the second working fluid inlet 32 is therefore referred to herein as “relatively concentrated second working fluid,” and the second working fluid 36 inside the absorber and exiting at the second working fluid outlet 34 is referred to herein as “relatively diluted second working fluid.”

The divider 16 can be disposed between, and in fluid communication with, the evaporator 12 and the absorber 14. The first side 18 of the divider 16 therefore may be contacted by liquid first working fluid 26 and the first working fluid vapor 28, and the second side 20 may be contacted by the second working fluid 36 and first working fluid vapor 28 that may be disposed in the absorber 14. In some embodiments, virtually all of the volume within the evaporator 12 may be occupied by liquid first working fluid 26 and/or all of the volume within the absorber 14 may be occupied by the second working fluid 36. In such cases, first working fluid vapor 28 in the evaporator 12 would be found mainly at the pores 38 of the divider 16.

The divider 16 may be configured to allow first working fluid vapor 28 to pass through between the first and second sides 18, 20, and to inhibit movement of liquid first working fluid 26 and the second working fluid 36 through between said first and second sides. For example, the divider 16 may define holes 38 there through. The holes 38 may be sized in accordance with the properties of the first and second working fluids 26, 36 and those of the material making up the divider 16 in order to assure that the interfacial energies of liquid first and second working fluids and the membrane are such that the liquid first and second working fluids are energetically prevented from assuming a configuration necessary to pass through the holes in the membrane.

The second working fluid 36 may be chosen such that, when received at the absorber 14 (under appropriate conditions), an equilibrium partial pressure P2 of first working fluid vapor 28 at the second side 20 (and possibly throughout the absorber) is less than a partial pressure P1 of first working fluid vapor at the first side 18 (and possibly throughout the evaporator 12). For example, the first and second working fluids 26, 36 may be chosen such that the second working fluid includes as a component thereof a liquid that has a strong affinity for the first working fluid. In such a case, the equilibrium partial pressure P2 of the first working fluid vapor 28 in the vicinity of the second working fluid 36 will tend to be low relative, for example, to the partial pressure P1 expected in the vicinity of liquid first working fluid 26. Examples of pairs of first and second working fluids 26, 36 that may be utilized in conjunction with embodiments of the above described system 10 include, but are not limited to, water and lithium bromide; NH₃ and water (or a mixture of water and NH₃); water and LiClO₃; water and CaCl₂, water and ZnCl₂; water and ZnBr; water and H₂SO₄; and SO₂ and organic solvents.

The difference in partial pressures P1 and P2 of first working fluid vapor 28 across the divider 16 results in a driving force for diffusion of first working fluid vapor from the first side 18 to the second side 20. Once first working fluid vapor 28 reaches the second side 20, it can be combined in the absorber 14 with the second working fluid 36, with this combination being made more likely by the proper choice of a second working fluid having an affinity for first working fluid. Mass of first working fluid 26 will therefore be transferred from the evaporator 12 to the absorber 14 in the form of a vapor 28.

As mass is transferred from the evaporator 12 to the absorber 14, the balance in the evaporator between liquid first working fluid 26 and first working fluid vapor 28 will be disrupted, resulting in reducing the temperature of the first working fluid or absorbing external thermal energy. It is noted that continued evaporation of liquid first working fluid 26 in the evaporator 12 does not necessarily require the input of external thermal energy, but instead may proceed simply due to the affinity of the second working fluid 36 for first working fluid. The evaporation will stop when the pressure of the first working fluid vapor 28 balances the saturated pressure of the first working fluid 26.

In the conventional chillers, the total pressure within either the evaporator or the absorber needs to be approximately the same as the respective partial pressure therein of first working fluid vapor. With the evaporator 12 and the absorber 14 separated by the divider 16 as discussed above in various embodiments of the invention, it may not be required that the total pressure within either of the evaporator or the absorber is approximately the same as the respective partial pressure therein of first working fluid vapor 28. Rather, the evaporator 12 may be configured such that the total pressure therein is greater than the partial pressure P1 of first working fluid vapor 28. In one embodiment, the total pressure at the evaporator is at least twice the partial pressure P1 of first working fluid vapor 28. Further, the absorber 14 may be configured such that the total pressure therein is at least twice the partial pressure P2 of first working fluid vapor 28. As such, embodiments of the system 10 may have a total size and weight that is significantly reduced with respect to conventional absorption chiller sub-systems.

For the first working fluid vapor 28 to be driven from one side of the divider 16 to the other, particular temperatures and pressures are desired. As mentioned above, the evaporation of liquid first working fluid 26 in the evaporator 12, the diffusion of first working fluid vapor 28 from the first side 18 of the divider 16 to the second side 20, and the absorption of first working fluid vapor (or other energy-releasing event) in the absorber 14 can proceed spontaneously, acting to transfer thermal energy from the evaporator to the absorber. However, as thermal energy is transferred, the temperature of the liquid first working fluid 26 (in the absence of any other energy transfers) may drop, thereby reducing (and eventually eliminating) the tendency for further evaporation. At the same time, the temperature of the second working fluid 36 (again, in the absence of any other energy transfers) will rise, thereby decreasing (and eventually eliminating) the tendency of first working fluid vapor 28 therein to be absorbed. It is noted that, in some embodiments, the divider 16 may include a thermally insulating material, thereby preventing the transfer of heat through from the absorber 14 to the evaporator 12.

In one embodiment, the primary working fluid 26 exiting from the first working fluid outlet 24 of the evaporator 12 may be recirculated to the inlet 22 of the evaporator 12 through a recirculating loop 40. The recirculating loop 40 may recirculate the first working fluid to the evaporator through the first working fluid inlet 22 or through any other point in between the first working fluid inlet 22 and the first working fluid outlet 24 in the evaporator 14. For example, the recirculating loop 40 may join the evaporator 14 at any of the points 42, 44, 46 of the evaporator.

Generally, the temperature of the refrigerant in the evaporator decreases from the inlet to outlet. In one embodiment, the temperature of the recirculated liquid refrigerant is lower than the refrigerant temperature at the inlet 22 and higher than the refrigerant temperature at the outlet 24. In one embodiment, it is desirable that the recirculated refrigerant meet the refrigerant in the evaporator at a point where temperature of the recirculated refrigerant matches temperature of the refrigerant in the evaporator. As a result, more vapor can be transferred to the absorber in the beginning part of the absorber, leading to increasing the crystallization margin.

In one embodiment, in order to allow the transfer of vapors from the evaporator 12 to the absorber 14 to continue, the first working fluid 26 exiting the evaporator 12 through the recirculating loop 40 is brought into thermal communication with a thermal energy source, such as a primary heater 50. As used herein, “thermal communication” means the thermal energy transfer in between the primary heater 50 and the recirculating loop 40. In one embodiment, the thermal energy is transferred from the primary heater 50 to the recirculating loop 40. The thermal communication with the primary heater 50 increases the temperature of the recirculated first working fluid 26 entering the evaporator 12 (may be through any one or all the points 22, 42, 44, 46). The temperature increase of the recirculated first working fluid 26 increases the partial pressure P1 of the first working fluid at the evaporator 12. The increased partial pressure P1 of the first working fluid at first side 18 further promotes the first working fluid vapor 28 transferring to the absorber 14 through the divider 16.

In one embodiment, the primary heater 50 is a heat exchanger 50 in fluid communication with a heat-source 60 as shown in FIG. 2. As used herein, the “fluid communication” means that the primary heater 50 and a heat source are connected through at least one fluid-passing conduit. In one embodiment, the primary heater 50 and the recirculating loop 40 are in fluid communication with each other. As used herein, a heat exchanger is a device, wherein thermal energy is transferred from one part to another. In one embodiment, a heat exchanger includes at least two fluid passing conduits, wherein the thermal energy is transferred from one conduit to another. The heat exchanger 50 may be configured to pass the heat from the heat-source 60 to the recirculating loop 40.

In one embodiment, the system 10 may further have a secondary heater 70 disposed in thermal communication with evaporator as shown in FIG. 3. Similar to primary heater 50, in one embodiment, the secondary heater 70 is a heat exchanger 70 in fluid communication with a heat-source 80 as shown in FIG. 4. In a further embodiment, the secondary heater 70 is in fluid communication with the evaporator 12. In one embodiment, the heat-source 60 that is in fluid communication with primary heater 50 and the heat source 80 that is in fluid communication with the secondary heater 70 are the same as shown in FIG. 4. In an instance of a same heat-source 60/80 supplying thermal energy to the primary heater 50 and the secondary heater 70, the primary 50 and the secondary 70 heat exchangers may have a parallel fluid communication with the heat-source 60/80 as shown in FIG. 4 or may have a series or successive fluid communication with the heat-source 60/80 as shown in FIG. 5. Depending on the design and operations requirement of the system 10, in an instance of successive fluid communication with the primary 50 and secondary 70 heat exchangers with the heat-source 60/80, the thermal communication may happen first with the primary heat exchanger 50 or the secondary heat exchanger 70.

In one embodiment, in order to allow the transfer of thermal energy from the evaporator 12 to the absorber 14 to continue, the absorber may be brought into thermal communication with a thermal energy sink (not shown). For example, a fluid stream (e.g., air or water) may be circulated between the absorber 14 and the thermal energy sink. The system 10 can therefore be used to extract thermal energy from the heat-sources 60, 80 and to deposit thermal energy at a thermal energy sink.

During operation of the system 10, a generator (not shown) may receive the relatively diluted second working fluid 36 that is outputted at the second working fluid outlet 34 of the absorber 14. As mentioned above, the second working fluid 36 that is outputted from the absorber 14 has been combined therein with first working fluid vapor 28 passing through the divider 16. The generator may be configured to receive the relatively diluted second working fluid 36 and to produce separate outputs of first working fluid vapor 28 and a relatively concentrated second working fluid 36. For example, thermal energy can be added at the generator to raise the temperature of the relatively diluted second working fluid 36, thereby driving some of the first working fluid dissolved therein out of the solution as first working fluid vapor 28. The remaining second working fluid, now being relatively concentrated second working fluid 36, can be directed back to the second working fluid inlet 32 of the absorber 14.

The first working fluid vapor 28 outputted from the generator may be directed to a condenser (not shown) to condense first working fluid vapor 28 and provide liquid first working fluid 26 to the evaporator 12. For example, thermal energy may be removed at the condenser, for example, through the use of a heat exchanger, to cause the first working fluid vapor 28 to condense.

Overall, the evaporator 12, absorber 14, generator, and condenser may operate to form a continuous cycle in which the second working fluid 36 is combined with first working fluid 26 at the absorber and separated from first working fluid at the generator, and first working fluid is converted from vapor to liquid at the condenser and from liquid to vapor at the evaporator.

As described earlier, the use of a membrane 16 between the evaporator 12 and absorber 14 may alleviate the need to maintain the total pressure in either of the evaporator or the absorber at a level that is about equal to the partial pressure of water vapor in either one. Specifically, each of the evaporator 12 and the absorber 14 may be configured such that a respective total pressure therein is greater than or equal to about atmospheric pressure. This may reduce the size, cost, and/or complexity of the evaporator 12 and absorber 14. In other embodiments either the evaporator 12 or the absorber 14 may be configured to operate either above or below atmospheric pressure and around ambient temperature.

EXAMPLE

The following example illustrates methods, materials, and results, in accordance with specific embodiments, and as such should not be construed as imposing limitations upon the claims. All components are commercially available, unless otherwise indicated.

In one example embodiment, the system 10 employs water as a first working fluid 26, and a solution of lithium bromide and water as the second working fluid 36 as shown in FIG. 3. The absorber 14 may be configured to combine water vapor 28 passing through the membrane 16 with the lithium bromide-water solution 36 entering at an inlet port 32, thereby forming in the absorber a lithium bromide-water solution 36 that is relatively diluted with respect to lithium bromide content (the solution previously being relatively concentrated with respect to lithium bromide content prior to being combined with water vapor passing through the membrane 16). Lithium bromide tends to have a strong affinity for water, such that the partial pressure of water vapor in the vicinity of lithium bromide tends to be relatively low and the diffusion of water vapor through the membrane 16 is facilitated. The dilution of the lithium bromide-water solution helps in reducing the crystallization of the lithium bromide at the absorber 14. The additional water vapor 28 formed at the evaporator due to the thermal communication with the heat exchangers 50/70 further helps in evaporation of water 26 at the evaporator and further reduces the lithium bromide crystallization at the absorber.

As described earlier, in one embodiment, the divider 16 is a membrane. The membrane 16 can define holes 38 that extend between the evaporator 12 and the absorber 14. The membrane 16, or at least the portions through which the holes 38 are defined, may be formed of substantially hydrophobic material (e.g., polytetrafluoroethylene, polypropylene, and/or polyvinylidene fluoride). By forming the holes 38 with a maximum diameter of about 100-500 nm from a substantially hydrophobic material, the movement of liquid water 26 through the membrane 16 is substantially prevented, while water vapor 28 is permitted to pass through the holes between the evaporator 12 and absorber 14. As mentioned earlier, in some embodiments, the membrane 16 may be formed of thermally insulating material, with examples being the hydrophobic materials listed above.

FIG. 5 shows a Dühring plot for the membrane-based system 10 of FIG. 3 with a primary heater 50 and a secondary heater 70. The plot depicts the crystallization curve of the lithium bromide solution 80, the water curve 82, the diluted lithium bromide curve 84, and the concentrated lithium bromide curve 86. In a conventional absorption chiller, the concentrated lithium bromide solution 86 absorbs the water vapor and follows the solid line 88 to become the diluted lithium bromide solution 84. In one embodiment of the present invention, the concentration of the lithium bromide solution gradually decreases as shown by the dotted line 90 and migrates towards the constant concentration line of the diluted lithium bromide solution 84. The evaporation process removes heat from the liquid water on the evaporator side, reducing the water temperature, while the primary heater 50 and the secondary heater 70 increase the temperature and thereby the evaporation rate of the water at the evaporator. The process continues until the refrigerant and solution reach their lowest temperatures and vapor pressures. In one embodiment, the dotted line 90 does not intersect line 88 at the diluted lithium bromide solution line 84. The dotted line 90 may intersect line 88 anywhere between the concentrated lithium bromide solution 86 and the diluted lithium bromide solution 84, depending on the desired crystallization margin.” Thus, in one embodiment, a decrease in the concentration of the lithium bromide solution may be slower than that depicted in FIG. 5. In this embodiment, after a certain dilution of the lithium bromide solution, the concentration of the lithium bromide solution may follow the path of the lithium bromide solution in a conventional chiller to reach the constant concentration line of the diluted lithium bromide solution.

The vapor pressure of the first working fluid in a conventional design is lower than that for the system 10 except for the very end section of the evaporator, described herein as shown in FIG. 6. The higher vapor pressure for the embodiments of the invention results from the higher temperature of the first working fluid for the invention. The increase of the vapor pressure of the example system 10 may result in the size reductions of the absorber and evaporator or the increase of the crystallization margin in the absorber.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 

1. A system comprising: an evaporator comprising a first working fluid inlet and a first working fluid outlet; an absorber comprising a second working fluid inlet and a second working fluid outlet; a divider having opposing first and second sides, wherein the first side is in fluid communication with the evaporator and the second side is in fluid communication with the absorber; a recirculating loop connecting the first working fluid outlet to the evaporator; and a primary heater disposed in thermal communication with the recirculating loop.
 2. The system of claim 1, wherein the divider comprises a membrane.
 3. The system of claim 2, wherein the membrane comprises a substantially hydrophobic material.
 4. The system of claim 2, wherein the membrane comprises a plurality of holes having an average diameter in a range from about 100 nm to about 500 nm.
 5. The system of claim 1, wherein the recirculating loop connects the first working fluid outlet to the first working fluid inlet.
 6. The system of claim 1, wherein the recirculating loop connects the first working fluid outlet to the evaporator at a point downstream of the first working fluid inlet.
 7. The system of claim 1, further comprising a secondary heater disposed in thermal communication with the evaporator.
 8. The system of claim 7, wherein the primary heater and the secondary heater are heat exchangers in fluid communication with a heat-source.
 9. The system of claim 8, wherein the heat exchangers are connected to the heat-source in a parallel fluid communication.
 10. The system of claim 8, wherein the heat exchangers are connected to the heat-source in successive fluid communication.
 11. The system of claim 7, wherein the primary heater and the secondary heaters are heat exchangers in fluid communication with different heat-sources.
 12. A system, comprising: an evaporator comprising a first working fluid inlet and a first working fluid outlet; an absorber comprising a second working fluid inlet and a second working fluid outlet; a hydrophobic membrane based divider having opposing first and second sides, wherein the first side is in fluid communication with the evaporator and the second side is in fluid communication with the absorber; a recirculating loop connecting the first working fluid outlet to the evaporator; a primary heater disposed in thermal communication with the recirculating loop; and a secondary heater disposed in thermal communication with the evaporator.
 13. A method comprising: evaporating at least a part of a first working fluid in an evaporator; passing at least a part of the first working fluid vapor through a divider to an absorber comprising a second working fluid; exiting at least a part of the first working fluid through a first working fluid outlet of the evaporator; heating at least a part of the first working fluid after departing from the first working fluid outlet; and recirculating at least a part of the heated first working fluid back to the evaporator.
 14. The method of claim 13, wherein the heated first working fluid is recirculated to a first working fluid inlet of the evaporator.
 15. The method of claim 13, wherein the heated first working fluid is recirculated to the evaporator at a point downstream of a first working fluid inlet.
 16. The method of claim 13, wherein the first working fluid comprises water.
 17. The method of claim 13, further comprising heating the first working fluid inside the evaporator.
 18. The method of claim 13, further comprising combining the first working fluid vapor with a second working fluid in the absorber.
 19. The method of claim 18, further comprising cooling at least a part of the second working fluid in the absorber.
 20. The method of claim 18, wherein the second working fluid comprises a lithium bromide solution. 